(a) Technical Field
The present invention relates to an organic light emitting device (OLED).
(b) Discussion of the Related Art
Trends toward lightweight and thin personal computers and televisions sets also require lightweight and thin display devices, and flat panel displays satisfying such requirements have been substituted for conventional cathode ray tubes (CRT).
The flat panel displays include, for example, a liquid crystal display (LCD), a field emission display (FED), an organic light emitting device (OLED), and a plasma display panel (PDP). OLEDs consume low power, and have a fast response time, a wide viewing angle, and a high contrast ratio.
An OLED is a self-emissive display device that includes two electrodes and an organic light emitting layer interposed between the two electrodes. One of the two electrodes injects holes and the other of the two electrodes injects electrons into the light emitting layer. The injected electrons and holes are combined to form excitons, and the excitons release energy in the form of emitted light.
Excitons are divided into singlet excitons and triplet excitons based on a spin state, and the singlet excitons mainly participate in light emission.
The efficiency of light emission is defined by the efficiency of a light emitting material, the internal quantum efficiency, and the external quantum efficiency. The internal quantum efficiency is a ratio of the number of photons generated in the organic light emitting layer to the number of charges injected from the electrodes, while the external quantum efficiency is a ratio of the number of photons emitted externally to the number of photons generated in the organic light emitting layer.
The internal quantum efficiency is defined by yield of the singlet excitons generated in the organic light emitting layer.
However, since the ratio of the generated singlet excitons and triplet excitons is quantum-statistically about 1:3, the maximum amount of the singlet excitons of the total generated excitons, which participate in the light emitting, is about 25%.